U.S. Pat. Nos. 3,909,241 and 3,974,245 to Cheney et al relate to free flowing powders which are produced by feeding agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles and collecting the particles in a cooling chamber containing a protective gaseous atmosphere where the particles are solidified.
U.S. Pat. No. 4,264,354 to Cheetham relates to producing spherical dental alloy powders by high frequency induction coil heating followed by cooling in a liquid medium.
Fine spherical metal particles such as iron, cobalt, nickel, chromium, and alloys thereof are useful in applications such as filters, precision press and sinter parts, and injection molded parts. Typical alloys include but are not limited to low alloy steels, stainless steels, tool steel powders, nickel and cobalt based superalloys. In such applications the powders are consolidated by standard methods such as hot or warm extrusion, PM forging and metal injection molding, or pressing and sintering.
Some of the better known processes for producing such metal powder particles are by gas or water atomization. Only a small percentage of the powder produced by atomization is less than about 20 micrometers. Therefore, yields are low and metal powder costs are high as a result and in the case of water atomization, the powder is often not spherical.
In European Patent Application No. WO8402864 published Aug. 2, 1984, there is disclosed a process for making ultra-fine powder by directing a stream of molten droplets at a repellent surface whereby the droplets are broken up and repelled and thereafter solidified as described therein. While there is a tendency for spherical particles to be formed after rebounding, it is stated that the molten portion may form elliptical shaped or elongated particles with rounded ends.
U.S. Pat. Nos. 4,711,660 and 4,711,661 relate to spherical particles and process for producing same by reducing the particle size of the material and high temperature processing followed by rapid solidification. The oxygen content of the spherical particles when the material is reduced in size by the preferred method of attritor milling is greater than about 0.8% by weight. It is desirable that the oxygen content be lower than this value because for better sintering and better mechanical properties, etc.